To facilitate bicycling over changing terrain, road conditions and weather conditions, bicycles commonly have multi-speed transmissions. One of the most common types of transmission systems for bicycles is the derailleur-type transmission in which the rear wheel and, in many cases, the pedal crank have associated multi-sprocketed assemblies whereby the gear ratio may be selectively adjusted by laterally moving the chain to engage with sprockets of varying diameter. In almost all commercial systems, the cyclist selects the gear ratio by manually operating one or more levers operably connected to the derailleur system. For a variety of reasons as outlined in U.S. Pat. No. 3,929,925, it would be desirable to have an automatic shifting device which adjusts the speed of the bicycle. While several automatic shifting devices have been proposed for bicycles or the like, see U.S. Pat. Nos. 4,201,094 and 2,956,443, no automatic shifting device has as yet achieved widespread commercial success. The need continues for automatic shifting devices which are sufficiently simple and reliable to gain commercial acceptance.